The present invention relates to hollow blow-molded containers of a biaxially oriented thermoplastic material, and more particularly to thin-walled plastic containers configured to accommodate partial evacuation without adverse effects on their appearance.
Lightweight, thin-walled containers made of thermoplastic materials such as polyester resin and thermoplastic polymers containing at least 50% by weight polymerized nitrile-group-containing monomer (hereinafter "nitriles") are well known in the container industry. For example, polyethylene terephthalate (PET) has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics, beverages and so on. PET can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a high molding accuracy. Strong, transparent and substantially heat resistant containers may be produced by the biaxial-orientation blow-molding process in which a tubular parison is oriented both laterally and longitudinally in a temperature range suitable for such orientation. Nitrile and heat-set PET containers are particularly heat resistant. Biaxially-oriented blow-molded containers have greater stiffness and strength as well as improved gas barrier properties and transparency.
As noted above, a tubular parison is generally utilized to make cylindrical or other shaped containers. When a cylindrical container is formed from a tubular parison, orientation and stretch levels around the circumference of the container are relatively uniform. However, when a non-cylindrical container is formed from a tubular parison, stretching problems occur during fabrication. Particularly in the base of the container, unequal stretching may result in unequal and not regularly repeatable shrinkage after the tubular parison is stretched into, for instance, a square cross-sectional shape. This problematical shrinkage is particularly undesirable in the bottom section of the container at the seating ring and up to the body section of the container, and results from highly stretched corners and less stretched middle sections and sides. This can result in the container rocking instead of sitting flat upon a shelf or the like, or having visible deformations. Similar though less extreme problems arise in the shoulder section of the container.
Also, when a thermoplastic container is filled with a hot liquid (such as a liquid sterilized at a high temperature) and sealed, subsequent thermal contraction of the liquid upon cooling results in a partial evacuation of the container which tends to deform the container walls and bottom section. Backflow into a filling mechanism and the use of vacuum filling equipment during filling operations can similarly create a partial vacuum inside the container resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, such as the unevenly stretched bottom section, resulting in an exaggerated irregular seating surface and commercially unacceptable appearance. This problem is exacerbated when the container body includes collapse panels, indented surface areas which provide for controlled, quantified collapse of the container upon evacuation.
By increasing the thickness of the container it is possible to some extent to strengthen the container and decrease the effects of vacuum deformation. However, increasing the thickness of the container results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant increased costs are not acceptable to the container industry. Additionally, even with increased container thickness, there still is uneven stretching around the bottom section of a non-cylindrical container.
A prior attempt to reduce the effects of vacuum deformation is disclosed in U.S. Pat. No. 4,355,728. This patent discloses a container having bulges in the bottom section to stabilize the container upon contacting a rest surface and also to provide endurance against elevated pressure within the container. A similar prior approach to reduce the effects of vacuum deformation in the bottom section of a container is disclosed in British Patent Specification No. 1,406,958.
Prior art approaches have included the use of outwardly extending bulges or radially inwardly extending ribs in the radially inner end portion of the bottom section of containers to accommodate controlled deformation and to eliminate rocking of the container upon a rest surface. However, these prior art approaches are of complex design and improvements therein are required.